For any user or manufacturer of cylindrical products such as tubes or bars, it is a common practice to inspect the parts for the diagnosis of variations in the wall thickness and the presence of flaws in the base material. For this purpose, ultrasonic inspection techniques have become a recognized standard in the industry. With such techniques, it is customary to employ a transducer to apply high-frequency acoustic energy into the cylindrical part to be tested. The high-frequency acoustic field is first pulsed through a coupling medium and into the inspected part. As the acoustic wave propagates through the tested part, it gets scattered by the encountered inhomogeneity and discontinuities of the material. A fraction of the acoustic field is consequently reflected back to a receiving transducer which detects the incoming acoustic field echo. The material characteristics along the sound path, such as wall thickness and possible flaws, are deduced by monitoring this returning signal.
A new paradigm was formed in the field of nondestructive testing with the introduction of phased array (PA) multi-elements ultrasonic technology to enhance the performance of conventional ultrasonic single element probe inspection systems. A general description of how phased array technology can be adapted to imaging systems is given in U.S. Pat. No. 5,563,346 with recent examples of applications for the inspection of spherically bounded materials and turbine blades described in U.S. Pat. Nos. 6,279,397 and 6,082,198 respectively.
Another application of phased array technology precisely amounts for the inspection of cylindrical parts during production. In order to ensure complete coverage of the concerned parts, it is necessary to place PA probes at positions that allow reaching the entire circumference of the cylindrical product. This may be accomplished either by placing the PA probes all around a non-rotating cylindrical part, or by having a rotating part to roll in front of a fixed PA probe. Typical phased array industrial inspection systems for rotating cylindrical parts currently comprise linear phased array probes that increase productivity significantly compared to conventional ultrasound systems. However, despite the obvious advantages of linear phased array probes with respect to primary axis electronic scanning and focusing, these probes lack the flexibility to provide optimal inspection results on a large range of tube and bar diameters and thicknesses without the need of changing either the PA probes or the probe holders. On one hand, changing these components between inspection cycles adds downtime and inactive equipment costs. On the other hand, employing a non-optimal phased array probe and probe holder for a given inspection results in poor signal to noise ratio levels and diminished near-surface resolution.
One group of existing efforts has relied on a PA probe that possesses ultrasound element spanning both the scan and elevation axes in the form of a bi-dimensional matrix. While the N columns of elements on the scan axis (primary axis) may be individually electronically addressed to allow for beam steering and focusing, the M rows of elements along the elevation axis (secondary axis) of a given column are connected in symmetrical pairs; in the following, the number of rows is assumed to be odd. This configuration, known in the art as 1.5 D array (see U.S. Pat. Nos. 5,490,512 and 6,089,096 for instance), obviously restrains the beam manipulation freedom along the elevation axis. Yet, the figure of merit of the 1.5 D array rests in its ability to form custom electronically focused beams along an otherwise passive axis while requiring only (M+1)/2×N electronic leads instead of M×N for a full 2 D array. It also permits the active control of the aperture size for field optimization as described in U.S. Pat. No. 5,846,201.
Another existing practice is known in the art to increase the signal to noise ratio of eventual flaws inside the inspection range. The optimal choice of aperture size may be drawn from empirical results or numerical simulations for instance. The preferred embodiment of the method is especially suited for the inspection of tubular parts, but may be used for the near surface inspection of whole cylindrical bars. This adapted aperture procedure is similar to the one found in U.S. Pat. No. 5,490,512.
Yet in another existing practice, the firing pattern is made concentrically to the cylindrical part. This configuration ensures that the pulsed acoustic field from each elemental transducer reaches the surface of the part with the same minimum time, thus minimizing the extent of the front wall echo and sharpening its boundaries. This type of firing setup is similar to the one found in US Patent Application No. 2006/0195273.
Accordingly, the objective of the present invention is to provide a method for inspecting an extended range of tube and bar diameters and thicknesses with a single probe and probe holder that provides the advantages of improved near-surface resolution (herein later also as NSR) and improved signal to noise ratio by exploiting the benefits of a 1.5 D phased array probe.